The industry has moved away form the use of pins as connectors for electronic packaging where the pins have a high cost of fabrication, unacceptable percentage of failed connections which require rework, limitations on I/O density and have the electrical limitations of the relatively high resistance connectors. Solder balls are superior to pins in all of the above features as well as being surface mountable, which has obvious implications given the increasingly small dimensions in the forefront technologies today.
Solder mounting is hardly a new technology, itself. But, the need remains to improve the solder systems and configurations in electronic structures. The use of solder ball connectors has been applied to the mounting of integrated circuit chips using the C-4 (controlled collapse chip connection) technology since the method and structure were first described and patented in U.S. Pat. Nos. 3,401,126 and 3,429,040 to Miller et al., which are assigned to the present assignee. A myriad of solder structures have since been proposed for the mounting of IC chips, as well as for interconnection of other levels of circuitry and associated electronic packaging.
Surface mount technology has gained acceptance as the preferred means of joining electronic devices together, particularly in high-end computers. As compared to more traditional pin connector methods, where a pin mounted to the backside of a ceramic module is thrust through a hole in the board, twice the number of modules can be placed at the same board area. Other advantages such as smaller component sizes, greater I/O densities, lower electrical resistance, decreased costs, and shorter signal paths have prompted the industry migration to surface mount technology.
A myriad of solder structures have been proposed for the surface mounting of one electronic structure to another. Typical surface mount processes form the solder structures by screening solder paste on conductive, generally metallic, pads disposed on a surface of a first electronic structure, or "substrate". A stencil printing operation is used to align the contact mask to the pads. The solder paste areas on the substrate are aligned to and placed on corresponding pads on a second electronic structure, or "board". In some processes, solder paste may alternatively or additionally be screened on the board pads. After placement, the substrate and board go through a reflow operation to melt the solder paste and create a solder bond between the corresponding pads on substrate and board.
Other known surface mount technologies use solder balls rather than a solder paste to provide the solder structures. By using solder balls, a more exact and somewhat greater quantity of solder can be applied than through screening. The solder balls are aligned and are held to the substrate and melted to form the solder joint on the conductive pads. As before, the substrate with the newly joined solder balls is aligned to the board. The solder balls are then reflowed to form a good solder bond between substrate and board.
However, both the solder paste and solder ball surface mount techniques suffer when the density of the pads increase. A certain quantity of solder must be maintained to assure a reliable solder joint. As the required quantity of solder becomes large relative to the pad spacing, solder bridging between non corresponding conductive pads becomes a problem. The bridging problem is accentuated by the greater solder amount which is molten during the reflow process.
However, the manufacture of a solder joint using both solder paste and solder balls has proven difficult. Solder balls are difficult to align and handle during the reflow process. Different methods using vibration, brushing and vacuum in association with an alignment plate have been proposed for dealing with solder balls alone. The addition of the solder paste further complicates the process. Many problems were encountered maintaining the solder ball centrality with respect to each other and on the substrate, even to the extend that the solder balls were missing entirely. With the misalignment of the solder balls, bridging between adjacent pad sites become a problem. Good physical contact between the solder balls, solder paste and substrate must be assured while simultaneously preserving the alignment between substrate pads and solder joints. Process time mushroomed as the number of process checks increased.
One module that can be manufactured with a combination solder ball/solder paste connection is a ceramic ball grid array (CBGA). CBGA modules are preferred in some situations because a higher density of I/Os can be packaged per unit area as compared with perimeter leaded components. Additionally, CBGAs offer other advantages. CBGAs are less susceptible to damage while handling and there tends to less induction noise than pin-in-hole devices.
However, there are some potential drawbacks to using CBGAs. The solder joints are under the CBGAs. Any repairs necessitate the complete removal of the CBGA module. It is difficult to inspect the I/O solder joints by conventional means. The repair process can be very time consuming and expensive. To minimize the need for repairs CBGA users can invest in a solder paste volume measurement tool to assure that every pad has the correct amount of solder. The use of such a tool can increase assembly cycle time.
A popular method of attachment currently in use requires that the solder paste be applied to the circuit card. The solder paste volume is then measured. The CBGA module and the circuit card are brought into contact the solder paste is reflowed.